by Larry Copeland, USA TODAY

by Larry Copeland, USA TODAY

Drivers in the nation's most-congested cities should add 38 to 95 minutes to many routine 20-minute trips if they want to arrive on time, warns a new study on gauging unpredictable traffic.

For the Washington, D.C., area, the study says, drivers should set aside nearly two hours - 114 minutes - to arrive on time 19 out of 20 times. For the Dallas area, it's 80 minutes and for San Diego, 58 minutes.

Fair warning, though: This applies only to the part of the trip on limited access roads, such as interstate highways, some toll roads and other roads that don't have traffic signals.

That's from a new mathematical index offered today by the Texas Transportation Institute in its annual Urban Mobility Report, which analyzes congestion patterns in 498 of the nation's urban areas to try to give motorists and shippers reliable driving times.

For the index, Institute researchers crunched data from INRIX, a traffic reporting service, the U.S. Transportation Department and state transportation departments.

"When you have enough information about what's happening every day of the year, you can ... tell people how much extra time they should allow," said Tim Lomax, a co-author of the report.

He said reliable trip times are important for motorists when they're going to the doctor, the airport or picking up a child at day care.

They're also important to truckers who transport goods. In 2011, trucks racked up $27 billion in congestion costs - and some of that was passed on to consumers, the report says.

The institute found that in 2011, congestion caused Americans to travel 5.5 billion additional hours and purchase an extra 2.9 billion gallons of fuel for a congestion cost of $121 billion.

While congestion is below the 2005 peak, researchers say, it's no cause for celebration: As the economy recovers, congestion will rebound.

The nation, Lomax said, "is missing out on an opportunity to create a better-performing transportation system." He said the nation should invest in roads, bridges and urban transit now while construction costs are relatively low.

For the fourth year in a row, the Washington area, including Northern Virginia, was the most congested urban area in the country. It was followed by Los Angeles, San Francisco and New York.

"Washington, D.C., has the dubious distinction of being number one in two areas," said Pete Ruane,president and CEO of the American Road & Transportation Builders Association. "It is the capital of partisan gridlock, and now traffic gridlock.

The institute's report, he said, "isn't a surprise to the millions of motorists who slog through bumper-to-bumper traffic in cities of all sizes, or to the businesses attempting to move their goods and services."